o-Quinonediazide compounds are known to undergo decomposition of the diazo group by irradiation with actinic light to form compounds having a carboxyl group. Thus, when imagewise exposed and developed with an alkaline aqueous developing solution, exposed areas of a light-sensitive layer containing an o-quinonediazide compound are removed, and the non-exposed areas are left to form an image. Therefore, such a light-sensitive layer has recently been frequently used as a so-called positive working light-sensitive component of a light-sensitive printing plate or so-called presensitized plate. Particularly, a light-sensitive layer comprising an o-quinonediazide compound mixed with an alkaline aqueous solution-soluble resin has been advantageously used from the practical point of view. Light-sensitive layers comprising an o-quinonediazide compound mixed with a novolak type phenol-formaldehyde condensate resin or a cresol-formaldehyde condensate resin are the most popular light-sensitive compositions used in the art. Useful developing solutions for light-sensitive layers containing these o-quinonediazide compounds include aqueous solutions containing sodium tertiary phosphate, sodium hydroxide, potassium hydroxide, sodium silicate, potassium silicate, ammonium silicate, etc., alone or in combination. However, an aqueous solution containing only sodium tertiary phosphate, sodium hydroxide or potassium hydroxide is not desirably used for developing a light-sensitive printing plate comprising an aluminum support because its aluminum-etching ability is too strong. In addition, they undergo serious deterioration of developing ability after repeated use, and a definite volume of the solution has an extremely small throughput capacity for processing light-sensitive printing plates.
Accordingly, the use of an alkali silicate aqueous solution has recently become popular because it shows a moderate etching action on aluminum and makes it possible to adjust the developing ability within some range by controlling the ratio of silicon oxide (SiO.sub.2) to alkali oxide (M.sub.2 O) (generally presented as a molar ratio of SiO.sub.2 /M.sub.2 O) and the concentration thereof. That is, as the content of the SiO.sub.2 increases, the developing ability is depressed with development stability being increased, whereas as the content of the M.sub.2 O increases, the developing ability is raised, with development stability being lowered. The phrase "development stability" as used herein means image stability against variation of developing time, and if the amount of M.sub.2 O only is increased, the image area which should reside on a support tends to be removed in a short time.
On the other hand, a developing solution comprising the alkali silicate aqueous solution is usually used by diluting a concentrated composition with city water or well water. In this occasion, particularly, when development processing with such the diluted developing solution is continuously conducted for a long time using an automatic developing machine, insoluble matter accumulates on the bottom of the tank in the developing machine or deposits in spraying pipes or nozzles, leading to abnormal function of the machine.